Most
primates, including humans, spend their lives in large social groups or
communities. In
the case of semi-terrestrial
species, such as baboons, being in a large community helps provide
protection against predatory cats, dogs, and hyenas. It also helps
protect scarce food resources. This is especially true for non-human primates
when the food is fruit. Leaf-eaters, such as colobus monkeys and
langurs, tend to form smaller social
groupings since there is little competition for their food. The very few
nocturnal species of primates are mostly small, relatively solitary hunters.
There are exceptions to these general patterns. For instance, the
vegetarian orangutans lead mostly solitary lives except for mothers with
their children before they go off on their own.

Most
non-human primate communities are more or less closed to contact with members
of other communities. Most often, they are tied to a particular locale
and rarely migrate outside of their home range. This aloofness from
other troops prevents high concentrations of individuals which could result in
rapid depletion of local resources. Communities usually
avoid each other and are aggressive towards outsiders. As a result,
social interactions between members of different troops are usually very
rare, especially for females. Chimpanzees are a notable exception.
When chimpanzees from different troops come together, there is often an exciting, friendly
encounter lasting several hours, following which, some of the adult females
switch groups. Apparently, they are seeking new mates.
Occasionally, however, contact between communities of the comparatively
unpredictable chimpanzees will develop into genocidal violence.

Mother- infant
subgroup
(patas monkeys)

Interactions
within non-human primate communities are usually unlimited.
Subgroups are rarely closed from group interaction. All members of a
community have daily face to face, casual communication. The most common
type of subgroup consists of a mother and her young offspring.

In
some forest living primates, contact between groups of the same
species is in the form of a specialized territorial defense behavior.
Instead of avoiding each other, groups actively converge near their common
territorial border and make hostile displays. Howler monkeys, indris,
siamangs, and gibbons all produce exceptionally loud vocalizations for this
purpose. This is a ritualized, essentially harmless form of aggression that is
intended to intimidate members of the neighboring community. All four of these
species live in home ranges that are usually so small that
the food resources of neighboring territories can be seen and become attractive.

Non-human Primate Social Group Composition

While
there is considerable variation in social group composition among the
primates, there
is very little variability within each species. In fact, most non-human primate species are limited to only one
of the following six basic patterns:

1.

single female and her offspring

4.

one-male-several-female group

2.

monogamous
family group

5.

multimale-multifemale group

3.

polyandrous
family group

6.

fission-fusion society

Humans are an exception in
that we form a variety of social group patterns. However, each human
society usually defines one of them as being acceptable and condemns the
others. Only the multimale-multifemale group
pattern is not normally found in any human society.

Single
Female and Her Offspring

The
single female and her offspring group pattern is rare for primates but common
for other mammals. It is found among the orangutans and some of the
small nocturnal prosimians (e.g., mouse lemurs and galagos). The adult
males lead their lives mostly alone. However, they come together with
females occasionally for mating. The males of these species generally
have large territories that overlap those of several females. Both
male and female children usually leave their mother when they reach sexual
maturity.

Single
female and
her offspring type
of family group

(orangutan
mother
with child carried
on her back)

Monogamous Family Group

Monogamous
groups consist of an adult male and female with their children. When
they are grown, the children leave to create their own
nuclear
families.
While this group pattern is the most common one for humans, it is rare for
non-human primates. It is found among the small Asian apes as well as some of the
New World monkeys and prosimians. Specifically,
monogamous family groups are the common pattern for gibbons, siamangs, titi monkeys, indris, tarsiers, and apparently some
pottos.

Polyandrous Family Group

The
smallest New World monkeys, the marmosets and tamarins, form both monogamous
and polyandrous
family units. They generally start with a monogamous mating pair.
Later, a second adult male may join
the family and assist in child rearing. When
this occurs, both adult males will potentially mate with the adult female.
This arrangement is practical because these monkeys commonly have twins and
the fathers carry the babies around on their backs most of the time.
This polyandrous mating pattern is extremely rare among non-human primates but
does occur in some human societies in
isolated rural regions of India, Sri Lanka, and especially Nepal, and Tibet.

Polyandrous
type of
family
group

(three pygmy
marmoset adults
sharing child
rearing chores)

One-Male-Several-Female Group

One-male-several-female groups have
polygynous
mating patterns. That is to say, one male
regularly mates and lives with more than one female at a time. Polygyny is generally not a promiscuous mating
pattern. Rather, the male and his female mates form a distinct mating
and child rearing group. This pattern is found among hamadryas
baboons, geladas, langurs, howler monkeys, gorillas and many human
societies. It has been a culturally preferred marriage pattern in
numerous Native American, African, and South Asian cultures. However,
polygyny is not as common among humans as monogamy, even in cultures that
advocate it.

It would
be a mistake to automatically assume that non-human primate one-male-several-female groups are dominated by males. Among
geladas, females
largely control the social group. This is despite the fact that the
males are larger, stronger, and more aggressive. Mothers, sisters, and
aunts act as a team in chasing off other unrelated females. They also
collectively select their mutual mate among a number of potential suitors
roaming in and out of their territory. The male that is chosen usually
is one that does not act abusively towards them and is willing to cooperate
with them in defending their territory. The relationship with any
particular male may be short-term. The stable core of the community is
the group of related females. This is a long way from stereotypical male
domination.

One-male-several
female groups may take a different form when predator pressure is a
problem. In open grasslands, hamadryas baboon communities are
much larger, usually consisting of a number of polygynous families. In
such multiple one-male-several-female group societies, males are the dominant,
controlling
members. The adult males not only "herd" their own sexually
mature females, but also maintain order and protect the community from
predators. This is not unlike the traditional Arab polygynous marriage
pattern in which wealthy men acquire harems.

In contrast, gorillas
rarely have to be concerned about predator dangers. Subsequently, their
communities usually consist of a single dominant adult male, his mates, and their
children. When males reach maturity, they usually are driven off by the
dominant silverback male. These exiled males ultimately form their own
one-male-several-female groups. As females reach sexual maturity, they
also leave their natal families and disperse. They later join with
single males to form new families or they join the families of males who
already have mates. When the silver back males have unusually peaceful
personalities, the gorilla community may have several of them.

NOTE:
The term "silver back" refers to older mature gorilla males who
normally develop large silvery-gray saddles of hair on their backs.
As they age, the saddles become larger and eventually cover most of
their backs.

Multimale-Multifemale Group

The
most common social group pattern among semi-terrestrial primates is the
multimale-multifemale group. With this pattern, there are no stable
heterosexual bonds--both males and females have a number of different
mates. This is characteristic of savanna baboons, macaques, as well as
some colobus and New World monkey species.

Multimale-multifemale
groups commonly have a dominance hierarchy among both males and females. Each individual is ranked relative to all other community members of the same
gender. This tends to reduce serious violence within the community since
everyone knows in advance who they must defer to and who must be submissive to
them. Among rhesus macaques, one's position in the dominance
hierarchy is determined by the rank of his or her mother. The top
ranking individuals are referred to by primatologists as the
alpha male
and the alpha female. All other community members defer to them. A female's rank
in the hierarchy stays with her throughout life. However, most young
adult male rhesus macaques leave their natal community and ultimately join others to find
mates. When they do so, they start at the bottom of the male dominance
hierarchy again. Alpha males usually mate more often than others.
This makes the social organization superficially look like
one-male-several-female group. However, younger females often sneak off
to mate with males lower down on the dominance hierarchy. The stable core of rhesus macaque
communities is the group of female relatives. They stay within their
natal community throughout life and work as a team to defend it against other
females.

Advancing up the male dominance hierarchy
in baboon communities is comparatively less peaceful than among macaques.
Adult male baboons must constantly face challenges from other males who
would take away their mates. These confrontations are usually noisy
and violent. The fights often result in serious injuries caused by
their exceptionally long canine teeth. The higher a male ascends the
dominance hierarchy, the more challenges he must answer. As a
consequence, alpha males are often under greater physical and psychological
stress than lower ranked ones. This results in much higher levels of
glucocorticoid stress hormones in their blood, which can result in impaired
immune systems. The upside of being a baboon alpha male is more sexual
access to females. The downside is a highly stressful life.
Health problems resulting from social stress are not limited to males, as
the following video shows.

A fission-fusion society is one in
which the social group size and composition change throughout the year with
different activities and situations. This is the social pattern
typical of chimpanzees. Individuals enter and leave communities from
time to time. Adult males occasionally wander off and forage alone or
join a few other males in a hunting party. Females casually change
membership from one group to the other. This occurs especially when
females are in estrus
and seeking mates. As a result, foraging and sleeping groups reform
frequently. Male chimps are the relatively stable core of the
community since they rarely join other troops.

What allows for the generally
loose relationship between chimpanzee communities is that they apparently
recognize a wider range of social bonds than do monkeys. They often
have relatives and friends in several different neighboring troops.
When chimpanzee communities come together, they usually exchange friendly
greetings rather than show aggression. However, it would be a mistake
to assume from this that chimpanzee society is always peaceful. The
adult males within each community are frequently engaged in complex
political activities involving scheming and physical intimidation in order
to move up the dominance hierarchy. They develop short-term alliances
with other males by mutual support, sharing meat, and allogrooming (grooming others). It isn't always the
largest and strongest males who make it to the top of the hierarchy.
Often teamwork used to frighten and impress is more effective than any one
individual's muscles in achieving chimpanzee goals. This is an
indication of their intelligence.

Chimpanzees are not the only
primates that change group membership from time to time. For instance,
most adult rhesus macaque males permanently leave the community of
their birth and try to join others in order to find mates. This is not
easy since they are not warmly welcomed in their adoptive
troop. Group composition of some langur and baboon species also change
as a result of the availability of food and mates. Evidently, none
of these monkey species change group composition with the ease and frequency
of chimpanzee females. As a result, their societies are not usually referred to
as fission-fusion types.

Social
Group
Pattern

Primate Species
Following This Pattern

single female and her
offspring

orangutans, some
of the
small nocturnal prosimians (mouse
lemurs and galagos), and some humans

monogamous family group

some
New World
monkeys (titi monkeys), some prosimians
(indris,
tarsiers, and some pottos), the small Asian apes
(gibbons, siamangs), and some humans

polyandrous
family group

the
smallest New World monkeys (marmosets and tamarins)
and some humans

Kinship refers to relationships
that are recognized between individuals based on family ties. Among
humans, those ties are created by marriage and shared descent from
ancestors. Among non-human primates, they are due to descent.
Most
non-human primates apparently only recognize matrilineal descent.
That is,
they know who their mother is but not their father. Socially recognized
paternity is unimportant or non-existent for them. The strongest social unit is a
mother and her young children. Chimpanzees maintain their bonds with
their mother well into adulthood. When they are threatened by others,
even large male chimpanzees may go to their aged mother to be soothed by
grooming. As mentioned earlier, the social ranking of juvenile rhesus
macaques precisely corresponds to the ranking of their mothers. Shared
descent from the same female is also the basis of close relationships
between macaque sisters, aunts, and nieces. Another
indication of the recognition of matrilineal descent is the fact that incest
in the form of mother-son mating is rare for primates.

Gender Differences

Extreme
sexual dimorphism
among adult mandrills
(female
on the left, male on the right)

Behavioral
differentiation of male and female primates involves more than just mating
behavior. Males are usually larger and physically dominant over
females. Extreme sexual dimorphism
is particularly characteristic of the
semi-terrestrial monkeys and the great apes. The manner in which male and female
children are socialized often differs. Following infancy, age-based play
groups are usually the primary socializing group for males. These
juvenile male cohorts often spend much of their time on the fringes of the
troop territory in active physical play. For example, nearly half of the waking
hours of young male gorillas are spent in rough play. This is comparable
to the amount of time human children play if given the opportunity. For
most non-human primates, play is critical for developing social bonds and
social skills. Young female monkeys and apes are more often socialized in the
safer areas of the community territory by their mothers and
other older female relatives. Their play is usually less rough and
focuses more on developing parenting skills and social ties among the females
with whom they usually spend their lives. As adults, male primates are more likely
to leave the community to join roaming bachelor groups and eventually join
other communities. Chimpanzees are somewhat of an exception. While
male chimpanzees often roam, it is the females who switch communities.
However, they usually stay physically close to their own mothers until they
become sexually mature around 11 years of age or somewhat later.

Within their own communities, male
chimpanzees spend much of their time scheming in order to move up in
dominance. The successful ones usually accomplish this by forming temporary
alliances with a few other males in order to physically dominate the rest.
These alliances are often created by sharing meat and other prized foods as
well as grooming each other. Female chimpanzees apparently do not take
part in this constantly changing, often violent political restructuring of
their community.

An even darker side of male chimpanzee
behavior is that they occasionally murder members of other chimpanzee
communities. Groups of males periodically go on aggressive raids into
neighboring territories where they isolate individual males and then
violently beat
and bite them to death. Over time, these marauding gangs will kill all
of the males in the targeted communities if they can. They also have
been observed eating the infants there, but they usually leave their mothers
alone. This behavior has been recorded at several locations in East
Africa. The motivations of the killers is not entirely clear, but the
net effect is to increase their territory and food resources and at the same
time reduce competition for potential mates. This kind of murderous
behavior is, of course, reminiscent of humans as well. There are many
instances of it in our own history.

NEWS:
In a November 21, 2006 Current
Biology report, Martin Muller
and his colleagues describe the mating patterns of common chimpanzees in Uganda's Kibale National
Park. Male chimps generally prefer to mate with females who are 30 years of age
or older. These older females are also more often fought over by
males. Even balding 55 year old females with worn and
broken teeth apparently are more appealing to males than are young healthy
females, aged 15-20. Muller speculates that the appeal of older
females is that they have been successful in surviving and in having and
raising offspring.